The invention relates to a coupling for electrically and mechanically connecting medium-voltage or high-voltage components, in particular for voltages of 1 kV to 52 kV.
FIG. 10 shows schematically a conventional medium-voltage installation comprising a switchgear cabinet 1, to which an electrical cable 2 can be connected. For this purpose, in the enclosure wall of the switchgear cabinet 1 is a bushing 3, which is designed in accordance with the technical standards DIN EN 50180 or DIN EN 50181 and comprises, for instance, a plug-in connecting piece 4 having an outer cone. The electrical cable 2 comprises at its unattached end a complementary connecting piece 5, which can be plugged onto the connecting piece 4 of the bushing 3 of the switchgear cabinet 1 to form a mechanical and electrical connection.
In medium-voltage installations of this type there is an increasing need to measure the current through the connection point between the two connecting pieces 4, 5. Inductive instrument transformers are known in the prior art for this purpose, but these transformers allow only a measurement with a relatively narrow bandwidth and therefore are not capable of measuring harmonic components as well.
Reference is made to US 2013/0183043 A1 regarding the general technical background. This document discloses a coupling for a mains voltage plug and/or a mains voltage socket. This known technology, however, cannot be applied to medium-voltage or high-voltage components.
Finally, reference is also made to DE 10 2011 113 002 A1, WO 2014/127788 A1 and DE 36 11 462 A1 regarding the general technical background.
Thus the object of the invention is to facilitate in medium-voltage and high-voltage installations a current measurement that is as simple as possible and also allows harmonic components to be measured.
This object is achieved by a coupling according to the invention as claimed in the main claim.
The invention is based on conventional couplings that are used in medium-voltage or high-voltage installations (e.g. for voltages of 1 kV to 52 kV) to connect together connecting pieces of components (e.g. electrical cables, bushings).
Such couplings conventionally comprise a first connecting piece, which can be connected to a medium-voltage or high-voltage component, for instance to a component such as a complementary connecting piece of a bushing of a switchgear cabinet.
Furthermore, such couplings conventionally comprise a second connecting piece to facilitate a connection to another medium-voltage or high-voltage component, for instance to a component such as a corresponding complementary connecting piece of an electrical cable.
It should be mentioned here that the two connecting pieces of the coupling according to the invention belong to complementary connecting pieces, which fit together mechanically and electrically. This has the advantage that the coupling according to the invention can be introduced easily between two complementary connecting pieces of a connection. To do this, it is merely necessary to separate the complementary connecting pieces of the connection from each other. Then subsequently the coupling according to the invention can be introduced between the two complementary connecting pieces and connected to these connecting pieces. The coupling according to the invention can thereby be inserted easily into an existing connection.
In comparison with conventional couplings, the coupling according to the invention is characterized in that a low-resistance current sensing resistor (also referred to as a “shunt”) is built into the coupling, which current sensing resistor is electrically connected between the two complementary connecting pieces so that the current flow between the two connecting pieces passes through the current sensing resistor, allowing a current measurement. This shunt-based current measurement also allows harmonic components of the current to be measured, as described later in detail below. Furthermore, by virtue of the shunt-based current measurement it is possible to dispense with an inductive instrument transformer, which is otherwise required. Finally, the coupling according to the invention allows contact to be made easily to the current sensing resistor in the current-flow path.
A measuring device is preferably also built into the coupling, which measuring device can measure, for example, the voltage drop across the current sensing resistor, from which the current through the current sensing resistor is then obtained using Ohm's law in a manner known per se.
There is also the facility, however, for the measuring device to measure the voltage at a measuring point on the current sensing resistor with respect to a reference potential (e.g. ground potential).
In a preferred exemplary embodiment of the invention, however, the device measures both values, namely the voltage drop across the current sensing resistor and the voltage at a measuring point on the current sensing resistor with respect to ground potential.
It should be mentioned here that the measuring device measures the voltage concerned preferably at a high sampling rate, which preferably equals at least 100 Hz, 200 Hz, 500 Hz, 1 kHz, 1500 Hz, 2 kHz, 5 kHz, 10 kHz or even 15 kHz. A sampling rate at such a high level also allows the measurement of harmonic components of the voltage, which is important in power supply systems.
It must be taken into account for the measurement of the voltage at the current sensing resistor that the measured voltage lies in the medium-voltage or high-voltage range and is therefore difficult to process using instrumentation. In the preferred exemplary embodiment of the invention, the measuring device therefore comprises a potential divider, which is connected to the measuring point on the current sensing resistor in order to divide the measured voltage and thereby simplify the use of instrumentation for processing. The potential divider preferably comprises a plurality of Ohmic resistors or capacitors and has a division ratio of greater than 1000, 10,000 or greater even than 100,000.
It should also be mentioned that the coupling according to the invention preferably allows galvanic isolation between the coupling at the medium-voltage potential or high-voltage potential on the one side and the external evaluation unit at ground potential on the other side. This makes it difficult to transfer the measurement data from the measuring device to the external evaluation unit via a data line. Thus the coupling according to the invention preferably comprises a data transfer device containing an electrical-to-optical transducer, which converts the electrical measurement data to be transferred into optical signals, which are then output to a first optical waveguide connector. A first optical waveguide can be connected to the first optical waveguide connector, via which the measurement data is then transferred to the external evaluation unit.
The data transfer device of the coupling preferably comprises a transmit buffer in order to buffer the measurement data for transmission before a transfer. This is advantageous because the aforementioned electrical-to-optical transducer has a relatively high power consumption and therefore is switched on and then off again preferably only intermittently in order to minimize a time-averaged power consumption. In the OFF phases of the electrical-to-optical transducer, the measurement data obtained in the coupling is written to the transmit buffer. In the ON phases of the electrical-to-optical transducer, the buffered measurement data is read from the transmit buffer and transferred.
The measurement data can be transferred in the form of messages, for example, each of which contains a plurality of data records, which each correspond to one specific measurement time instant. In this context, the transfer rate of the individual successive messages is preferably far lower than the sampling rate used to perform the actual measurement. As a consequence, messages are transferred only relatively infrequently, and therefore the electrical-to-optical transducer needs to be switched on only relatively infrequently, resulting in a correspondingly low average power consumption since the electrical-to-optical transducer is the main power consumer in the coupling.
In addition, the coupling according to the invention preferably also comprises a built-in power supply device for supplying power to the measuring device and/or to the data transfer device. It has already been mentioned above that the measurement data is transferred in a galvanically isolated manner. It is therefore also advantageously provided that the power is supplied in a galvanically isolated manner by supplying optical energy. For this purpose, the coupling according to the invention can comprise a second optical waveguide connector for connecting a second optical waveguide, via which the optical energy can be supplied, which optical energy can be generated by an external laser, for example. In this case, the coupling contains a photovoltaic cell for converting the supplied optical energy into electrical energy for the power supply, wherein the photovoltaic cell is connected to the second optical waveguide connector in the coupling.
It should be mentioned in this context that the supply of power by means of optical energy allows only a relatively low supply of energy compared with power supplied via electrical lines. It is therefore advantageous if the electrical-to-optical transducer, as the main power consumer, is switched on/off only intermittently, as already mentioned above, because this reduces the average power consumption.
The power supply device preferably comprises an energy storage device for buffering the supply of power in order to be able to provide enough electrical power even when the electrical-to-optical transducer is in the ON state. This energy storage device may be, for instance, a capacitor, which preferably has a capacitance of at least 500 mF or 1 mF.
With regard to the mechanical design of the coupling according to the invention, it should be mentioned that the coupling preferably comprises a housing, which contains a control electrode made of an electrically conductive plastic, wherein the control electrode conducts the field lines of the electric field and can accommodate electronic components in its interior, for instance components such as the measuring device, the power supply device and/or the data transfer device.
In addition, the housing of the coupling can comprise an encapsulation made of an electrically conductive plastic, wherein the encapsulation contains the control electrode and the electronic components.
There can be electrically insulating silicone potting between the encapsulation and the control electrode.
In addition, the housing can comprise an electrically conductive enclosing wall, which can be made of aluminum, for example, in particular in the form of two half-shells, inside of which the electrically conductive enclosing wall preferably comprises an electrical contact, which is connected both to the potential divider and to a ground contact.
With regard to the connecting pieces of the coupling according to the invention, it should be mentioned that said connecting pieces are preferably plug-in connecting pieces having an inner cone or respectively an outer cone, as specified in the technical standards DIN EN 50181 and DIN EN 50180, for example. The connecting pieces can conform to interface type A, B or C of the above-mentioned technical standards, although in principle the invention also includes other interface types.
It must also be mentioned that the coupling according to the invention preferably is suitable for electrical currents of greater than 250 A and voltages of greater than 25 kV.
It has already been mentioned above that the coupling according to the invention contains a low-resistance current sensing resistor, where the resistance value of the current sensing resistor is preferably less than 1 mΩ, 500μΩ, 250μΩ, 100μΩ, 50μΩ or even is less than 25μΩ.
With regard to the structural design of the current sensing resistor, it is possible to have a rotationally symmetric shape or a planar shape, as disclosed by EP 0 605 800 A1.
With regard to the heat capacity of the current sensing resistor, it should be mentioned that the heat capacity is preferably at least 50 J/K, 100 J/K, 200 J/K or even is greater than 300 J/K. A heat capacity of such a high value is advantageous, because then, even under a heavy current load, the current sensing resistor heats up only slightly, avoiding measurement errors. To achieve this, the current sensing resistor must merely have a correspondingly large mass in order to achieve the desired thermal buffering.
It should finally be mentioned that the invention claims protection not only for the coupling described above as a single component, but also claims protection for a corresponding medium-voltage or high-voltage installation containing such a coupling that connects together two medium-voltage and/or high-voltage components (e.g. switchgear cabinet and electrical cable).
This installation can also contain a light source (e.g. laser), which is then used to supply power to the coupling according to the invention.
Furthermore, the installation according to the invention can also contain an evaluation unit, which is connected via an optical waveguide to the coupling according to the invention in order to receive the measurement data.